Based on satellite images, they've determined that the North is working on a missile with a booster so large that it could fire more than 6,200 miles, meaning it would be capable of reaching the continental US.

The officials aren't sure if the missile is functional, or "a life-sized mockup," but they suspect it will be unveiled either during a military parade on April 15, or during the army's April 25 centennial celebration.

The North has around 10,000 missile experts with "considerable research skills," a source said in a separate report, and has spent some $3.1 billion on developing long-range ballistic missiles so far. For reference, South Korea has about 3,000 rocket experts total between the public and private sectors.